Paper is composed of cellulosic fibers that are formed into a sheet. The majority of paper making machine consists of three main sections: the forming section, the pressing section, and the drying section.
In the forming section, the cellulosic pulp slurry or furnish is injected onto a forming fabric which is a long, woven mesh belt. As the forming fabric moves along through the forming section, some of the water in the slurry drains through the fabric and a paper web is formed. As this paper web leaves the forming section, it is composed of about 80% water and about 20% solids.
For many years, forming fabrics were woven from metal wires and had a life of about one week on a paper machine. This short life was due to metal fatigue and abrasion caused by contacting the machine parts in the forming section.
In the 1960's, experiments were begun to replace the metal fabrics with woven, synthetic, monofilament yarn fabrics. Today, polyester monofilament is the yarn of choice for this application and typical fabric life is about 60-120 days.
After leaving the forming section, the paper web moves into the pressing section where a high compressive force is exerted by a pair of press rolls to remove more water from the paper web. The press fabric serves as cushioning and water removing media between the press rolls. As the paper leaves the pressing section, the paper web contains about 60% water and 40% solids.
Traditionally, press fabrics were made of 100% wool due to its resilience and water absorbency. However, synthetics have been developed with good resilience that have longer life than woolen felts. Fabrics of choice today consist of a base fabric, woven from polyamide monofilaments into which polyamide fibers have been needlepunched to form a felt. Typically life of press felts is 30-60 days.
The drying section consists of large, steam-heated cylinders that dry the paper web to a level of about 6% moisture.
A dryer felt or fabric is needed to hold the paper in contact with the dryer cylinders. Originally, these fabrics were made from cotton, but as paper making developed, higher speed and temperature shortened the life of the cotton dryer felts.
Many different fibers and yarns have been used to develop better-performing dryer felts so as to improve the efficiency of the paper making process. Presently, the predominant yarn used in the manufacture of dryer fabrics is polyester monofilament. See, Luciano, B., Albany International Fabric Facts, Volume 38, No. 4-6. Dryer fabrics made from polyester monofilament operating at normal temperatures (300.degree. to 350.degree. F.) last about one year.
In order to improve profitability, paper makers desire to increase speeds of the paper making machines. To sufficiently dry the paper at increased throughput, additional heat is used in the dryer section and perhaps in other sections of the paper machine as well.
Elevated temperatures tend to adversely affect the hydrolysis resistance of polyester yarns. For this reason manufacturers of dryer fabrics have looked at other fibers and yarns in an effort to increase fabric life at higher temperatures.
Moreover, if a fabric has to be replaced at other than scheduled maintenance cycles due to failure or damage, the downtime cost to the paper maker can be significant. For this reason, it is desirable to manufacture dryer fabrics that will run with longer and more predictable times under increased heat and speed conditions.
As we move into the next century, an ever increasing emphasis is being put on using recycled paper in the making of new paper. Federal and state laws are being passed which require a certain amount of recycled paper to be used in each pound of paper manufactured. For the environmentalist this is a good law, however, for the paper makers this law poses new challenges because the recycled paper has a high level of contaminants. Contaminants include wood pulp residues, inorganic residues (such as clays and titanium dioxide), adhesives from mailing labels, stickers from hot-melt adhesives, non-paper films, and printing inks. These contaminants may either stick to the paper making fabrics or be carried on through the paper machine in the paper sheet. If these contaminants cannot be easily removed, the fabrics will become plugged and the quality of the paper will decrease to the point that the fabric must be replaced. Due to the ease of cleaning, fabrics made from 100% monofilaments are desired. See: Luciano, B., Ibid.
One solution is to use polyphenylene sulfide (PPS) monofilaments in the manufacture of dryer felts. PPS has very good hydrolysis resistance, but unfortunately, the polymer is difficult to extrude into monofilaments and is quite expensive. Also, PPS monofilaments are very brittle which can cause problems on the paper machine. An example of a PPS monofilament is found in U.S. Pat. No. 5,162,151, which is incorporated herein by reference.
Another fiber solution to the harsh environment of the paper making process is the use of poly(2-methyl-1,5-pentylene) terephthalamide. See U.S. Pat. No. 5,162,152, which is incorporated herein by reference. Yet another fiber solution is the use of a copolymer of terephthalic acid, isophthalic acid, and 1,4-dimethylocyclohexane (also referred to as 1,4-cyclohexanedimethanol). See: U.S. Pat. No. 5,169,499, which is incorporated herein by reference.
Accordingly, there is a need in the paper making industry to develop new fibers for use in paper making clothing.